I. Field of the Invention
The present invention relates to the field of acoustic transducers. More specifically, the present invention relates to microfabricated transducers formed on the same chip as other integrated circuit components and a method for making the same.
II. Description of the Related Art
An acoustic transducer is an electronic device used to emit and receive sound waves. Acoustic transducers that operate at frequencies beyond the range of human hearing are used in medical imaging, non-destructive evaluation, and other applications. The most common forms of acoustic transducers that operate in the ultrasonic frequency range are piezoelectric transducers.
In a typical ultrasonic medical imaging system, an acoustic transducer is used along with other components, such as amplifiers, analog to digital converters, digital to analog converters, switches, analog multiplexors, digital multiplexors, microprocessors or microcontrollers. Different types of transducers, such as capacitive microfabricated transducers, can be used in systems that usually use piezoelectric components. In these systems, the transducers used are connected to certain other components via cables that electrically connect the transducer to the appropriate components.
Microfabricated transducers that are used in systems such as mentioned above will include, as illustrated in FIG. 1A, a conventional microfabricated transducer 10 that contains a void region 12 covered by a membrane 14. Disposed on top of the membrane 14 is one electrode 16 of a capacitor, and disposed below the void region 12 is another electrode 18 of a capacitor.
In operation, such a transducer can be used to generate an acoustic signal or detect an acoustic signal. By generating electrical signals on the electrodes of the transducer, an electrostatic attraction between the electrodes 16 and 18 is caused. This attraction causes oscillation of the membrane 14, which, by thus moving, generates the acoustic signal. Similarly, an incoming acoustic signal will cause the membrane 14 to oscillate. This oscillation causes the distance between the two electrodes 16 and 18 to change, and there will be an associated change in the capacitance between the two electrodes 16 and 18. The motion of the membrane 14 and, therefore, the incoming acoustic signal can thus be detected.
Improvements in the sensitivity of microfabricated acoustic transducers have been proposed. One example is the acoustic transducer disclosed in U.S. patent application Ser. No. 09/315,896 filed May 20, 1999, entitled "Acoustic Transducer And Method Of Making The Same."
Arrays of acoustic transducers, whether integrated or not, are also known. In a typical acoustic transducer array, independent acoustic transducers are capable of being excited and interrogated at different phases. While having an array of acoustic transducers enables the imaging functionality, each independent acoustic transducer in the array must have distinct signal lines so that the signal that is to be generated and/or detected can be independently controlled. As the number of independent acoustic transducers in an array becomes large, the number of additional signal lines necessary to control the different acoustic transducers becomes very large, which can limit the ultimate size of the array. In the context of microfabricated acoustic transducer devices, since the number of available paths able to establish an appropriate electrical contact with the electrical circuit is limited, large arrays of microfabricated acoustic transducers have heretofore been unavailable.
Also heretofore unavailable have been single elements and arrays of acoustic transducers formed with other specific integrated circuits. PCT Application No. WO/98/19140, however, proposes placing a transducer on the same integrated circuit chip with other electronic components. FIG. 1B, taken from FIG. 1 of this PCT application, illustrates that the transducer is formed integrally with the other electronic components 13. Thus, for instance, the electrodes to which the terminal contacts 4,6 connect are formed on opposite sides of the cavity 8, integrally with portions of the other electronic components 13. Thus, for example, the lower electrode of the transducer is formed within the same substrate region as other adjacent electronic components 13. This approach to forming a transducer with other specific integrated circuits, however, requires that the resulting integrated circuit provide certain areas for the transducer, and other adjacent areas for the electronic components 13, which results in an integrated circuit that is extremely complex in its layout. Furthermore, in order to obtain a transducer in a reasonable number of fabrication steps, compromises to the design must be made, or the number of process steps needed will result in an extremely costly process. Accordingly, this approach has drawbacks and this approach does not appear to be in widespread use.
Thus, it can be appreciated that while microfabricated transducers have many advantages, there are still many impediments to their widespread use. In addition to the difficulties noted above, it has been further recognized by the present inventor that making microfabricated transducers on their own separate substrate subjects the system to additional limitations. In particular, when the microfabricated transducer chip is connected to electronic circuitry, the electrical load (both real and imaginary) of such electrical connections and discrete electronics can negatively impact the performance of the transducer.
What is needed therefore, is a microfabricated transducer that can be formed, singly, in linear arrays, or in 2 dimensional matrices, over other integrated circuit components on the same chip, and a method for making the same